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-- Copyright (c)2013 Jeremy Seth Henry

-- All rights reserved.

--

-- Redistribution and use in source and binary forms, with or without

-- modification, are permitted provided that the following conditions are met:

--     * Redistributions of source code must retain the above copyright

--       notice, this list of conditions and the following disclaimer.

--     * Redistributions in binary form must reproduce the above copyright

--       notice, this list of conditions and the following disclaimer in the

--       documentation and/or other materials provided with the distribution,

--       where applicable (as part of a user interface, debugging port, etc.)

--

-- THIS SOFTWARE IS PROVIDED BY JEREMY SETH HENRY ``AS IS'' AND ANY

-- EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED

-- WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE

-- DISCLAIMED. IN NO EVENT SHALL JEREMY SETH HENRY BE LIABLE FOR ANY

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-- (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;

-- LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND

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--

-- VHDL Units :  o8_epoch_timer

-- Description:  Provides a 24-bit, 4uS resolution elapsed timer with

--            :   alarm and interrupt for the Open8 CPU.

--

-- Notes      :  Requires an externally provided uSec tick input - one clock

--            :   per microsecond.

--

-- Register Map:

-- Offset  Bitfield Description                        Read/Write

--   0x0   AAAAAAAA B0 of Setpoint (W) or Buffered Time(R)

--   0x1   AAAAAAAA B1 of Setpoint (W) or Buffered Time(R)

--   0x2   AAAAAAAA B2 of Setpoint (W) or Buffered Time(R)

--   0x3   DC----BA Control/Status register (RW)

--                  A = Update Buffered Time from internal timer (W)

--                  B = Reset Internal Epoch Time (W)

--                  C = Interrupt Enable (RW)

--                  D = Alarm State Flag (RW) (write a 1 to clear)

library ieee;

use ieee.std_logic_1164.all;

  use ieee.std_logic_unsigned.all;

  use ieee.std_logic_arith.all;

  use ieee.std_logic_misc.all;

library work;

  use work.open8_pkg.all;

entity o8_epoch_timer is

generic(

  Reset_Level           : std_logic;

  Address               : ADDRESS_TYPE

);

port(

  Clock                 : in  std_logic;

  Reset                 : in  std_logic;

  uSec_Tick             : in  std_logic;

  --

  Bus_Address           : in  ADDRESS_TYPE;

  Wr_Enable             : in  std_logic;

  Wr_Data               : in  DATA_TYPE;

  Rd_Enable             : in  std_logic;

  Rd_Data               : out DATA_TYPE;

  Interrupt             : out std_logic

);

end entity;

architecture behave of o8_epoch_timer is

  constant User_Addr    : std_logic_vector(15 downto 2)

                          := Address(15 downto 2);

  alias  Comp_Addr      is Bus_Address(15 downto 2);

  signal Addr_Match     : std_logic;

  alias  Reg_Addr       is Bus_Address(1 downto 0);

  signal Reg_Addr_q     : std_logic_vector(1 downto 0);

  signal Wr_En          : std_logic;

  signal Wr_Data_q      : DATA_TYPE;

  signal Rd_En          : std_logic;

  signal epoch_tmr      : std_logic_vector(25 downto 0);

  alias  epoch_tmrcmp   is epoch_tmr(25 downto 2);

  signal epoch_buffer   : std_logic_vector(23 downto 0);

  signal epoch_setpt    : std_logic_vector(25 downto 0);

  signal epoch_alarm    : std_logic;

  signal epoch_alarm_q  : std_logic;

  signal epoch_gie      : std_logic;

begin

  Addr_Match            <= '1' when Comp_Addr = User_Addr else '0';

  io_reg: process( Clock, Reset )

  begin

    if( Reset = Reset_Level )then

      epoch_tmr         <= (others => '0');

      epoch_buffer      <= (others => '0');

      epoch_setpt       <= (others => '0');

      epoch_alarm       <= '0';

      epoch_alarm_q     <= '0';

      Wr_Data_q         <= (others => '0');

      Reg_Addr_q        <= (others => '0');

      Wr_En             <= '0';

      Rd_En             <= '0';

      Rd_Data           <= (others => '0');

      epoch_gie         <= '0';

      Interrupt         <= '0';

    elsif( rising_edge( Clock ) )then

      epoch_tmr         <= epoch_tmr + uSec_Tick;

    -- Force the lower bits of the setpoint to "11" so that the offset is

    --  reduced to 1uS (reproducing the original behavior). Software should

    --  always subtract 4uS (-1) from the desired time to compensate

    epoch_setpt(1 downto 0) <= "11";

      Reg_Addr_q        <= Reg_Addr;

      Wr_Data_q         <= Wr_Data;

      Wr_En             <= Addr_Match and Wr_Enable;

      if( Wr_En = '1' and or_reduce(Reg_Addr_q) = '0' )then

        epoch_buffer    <= epoch_tmrcmp(25 downto 2);

      end if;

      if( Wr_En = '1' )then

        case( Reg_Addr_q )is

          when "00" =>

            epoch_setpt(9 downto 2)   <= Wr_Data_q;

          when "01" =>

            epoch_setpt(17 downto 10) <= Wr_Data_q;

          when "10" =>

            epoch_setpt(25 downto 18) <= Wr_Data_q;

          when "11" =>

            if( Wr_Data_q(0) = '1' )then

              epoch_buffer            <= epoch_tmrcmp;

            end if;

            if( Wr_Data_q(1) = '1' )then

              epoch_tmr               <= (others => '0');

            end if;

            epoch_gie                 <= Wr_Data_q(6);

            if( Wr_Data_q(7) = '1' )then

              epoch_alarm             <= '0';

            end if;

          when others => null;

        end case;

      end if;

      -- Set and hold on alarm condition

      if( (epoch_tmr > epoch_setpt) and (epoch_alarm = '0') )then

        epoch_alarm     <= '1';

      end if;

      epoch_alarm_q     <= epoch_alarm;

      -- Fire on rising edge of epoch_alarm

      Interrupt         <= epoch_gie and

                           (epoch_alarm and not epoch_alarm_q);

      Rd_Data           <= (others => '0');

      Rd_En             <= Addr_Match and Rd_Enable;

      if( Rd_En = '1' )then

        case( Reg_Addr_q )is

          when "00" =>

            Rd_Data     <= epoch_buffer(7 downto 0);

          when "01" =>

            Rd_Data     <= epoch_buffer(15 downto 8);

          when "10" =>

            Rd_Data     <= epoch_buffer(23 downto 16);

          when "11" =>

            Rd_Data     <= epoch_alarm & epoch_gie &

                           "000000";

          when others => null;

        end case;

      end if;

    end if;

  end process;

end architecture;